HRC-I/Chandra X-ray observations towards σ Orionis

Aims. We investigated the X-ray emission from young stars and brown dwarfs in the sigma Orionis cluster (tau similar to 3Ma, d similar to 385 pc) and its relation to mass, the presence of circumstellar discs, and separation to the cluster centre by taking advantage of the superb spatial resolution of the Chandra X-ray Observatory. Methods. We used public HRC-I/Chandra data from a 97.6 ks pointing towards the cluster centre and complemented them with X-ray data from IPC/Einstein, HRI/ROSAT, EPIC/XMM-Newton, and ACIS-S/Chandra together with optical and infrared photometry and spectroscopy from the literature and public catalogues. On our HRC-I/Chandra data, we measured count rates, estimated X-ray fluxes, and searched for short-term variability. We also looked for long-term variability by comparing with previous X-ray observations. Results. Among the 107 detected X-ray sources, there were 70 cluster stars with known signposts of youth, two young brown dwarfs, 12 cluster member candidates, four field dwarfs, and two galaxies with optical-infrared counterpart. The remaining sources were extragalactic. Based on a robust Poisson-chi(2) analysis, nine cluster stars displayed flares or rotational modulation during the HRCI observations, while eight other stars and one brown dwarf showed X-ray flux variations between the HRC-I and IPC, HRI, and EPIC epochs. We constructed a cluster X-ray luminosity function from O9.5 (about 18 M-circle dot) to M6.5 (about 0.06 M-circle dot). We found: (i) that early-type stars in multiple systems or with spectroscopic peculiarities tend to display X-ray emission; (ii) that the two detected brown dwarfs and the least-massive star are among the sigma Orionis objects with the highest L-X/L-J ratios; and (iii) that a large fraction of known classical T Tauri stars in the cluster are absent in this and other X-ray surveys. Finally, from a spatial distribution analysis, we quantified the impact of sensitivity degradation towards the HRC-I borders on the detection of faint X-ray sources and concluded that dozens X-ray sigma Orionis stars and brown dwarfs still need to be detected.